Cranes typically include counterweights to help balance the crane when the crane lifts a load. Since the load is often moved in and out with respect to the center of rotation of the crane, and thus generates different load moments throughout a crane pick, move and set operation, it is advantageous if the counterweight, including any extra counterweight attachments, can also be moved forward and backward with respect to the center of rotation of the crane. In this way a smaller amount of counterweight can be utilized than would be necessary if the counterweight had to be kept at a fixed distance.
A crane includes capacity charts developed by the manufacturer that specify a maximum weight a crane may lift with a given boom combination. Because a crane may be operated with a variety of boom combinations with varying lengths of boom components, a large number of capacity charts are required. For example, a simple crane having either a standard boom or luffing jib, five different lengths of booms, and five different jib lengths, would require thirty different capacity charts. Furthermore, each capacity chart would need to calculate the capacity of the crane for each distance from the center of rotation that a lift may occur.
When a variable position counterweight is used, the capacity is typically calculated with the variable position counterweight at its furthest extent, since this will result in the highest capacity for the crane. However, there are instances in which an operator may not want the variable position counterweight to extend to it greatest extent. For example, if an operator is operating the crane near a wall, the variable position counterweight may contact the wall if it were to be moved to its furthest extent.
Furthermore, sometimes a need arises in which it would be beneficial to lock or fix the position of the variable position counterweight in one location. This may occur for several reasons, including a desire to speed up cycle times by eliminating any delay that may occur while the variable position counterweight moves to its optimal position; to minimize the tailswing of the crane (e.g., such as when operating near an obstacle); to permit the crane with a variable position counterweight to operate as a traditional crane; to permit a crane operator the opportunity to affect the attitude of the crane and its effective ground bearing pressure (GBP); to accommodate for a mechanical failure in the actuating mechanism of the variable position counterweight that does not otherwise affect the safety of the crane; and to accommodate precision lifts during which movement of the variable position counterweight might have an effect on the placement of a lifted load.
For this reason, capacity charts are also generated for the counterweight at a position less than the maximum extent. A crane may have a variable position counterweight that extends nearly sixty feet from the center of rotation of the crane, but an operator may be interested in the capacity of the crane with the counterweight at a position less than sixty feet, such as at fifty feet. Since each position requires each of the load charts described previously to be recalculated, a limited number of positions are selected for generation of capacity charts. The crane with a variable position counterweight having a sixty foot maximum extent may choose three discreet positions for calculation of capacity charts, resulting in three times as many capacity charts as compared to a fixed counterweight.
If an operator needs to use the crane within a confined space, the operator must select either select a discrete position of the counterweight that is less than the available space, but that corresponds to a position on the capacity charts, or use the available space, but limit lifts to the capacity given for the discrete position of the capacity chart. Because a limited number of positions are selected for generating capacity charts, the intermediate position in the capacity charts may be substantially less than the available space. Using the previously example of a variable position counterweight have a sixty foot maximum extent and three discreet positions for calculating a capacity chart, each discreet position may be separated by twenty feet, with load charts at a twenty foot extent, a forty foot extent, and the maximum, sixty foot extent. If the operator needs to limit the counterweight to less than fifty feet, they would need to select a capacity corresponding to the counterweight position of forty feet. This results in a crane capacity that is substantially less than what would be available if the capacity were determined with the counterweight being able to extend to use all of the available space.
Crane operators would prefer to maximize the capacity their crane by having a large number of available intermediate positions used for calculating load charts. However, this substantially increase the amount of paper charts that must be maintained, the amount of data stored in the crane, and the number of calculations that must be performed. Thus, there is a need for providing a crane operator with crane capacity charts at a large number of discrete positions, while limiting the amount of paper capacity charts, data stored in the crane, and the total number of calculations required. In turn, these methods increase the flexibility and ease of use of cranes with a variable position counterweight system.